Experimental Studies Of Dynamic Constitutive Properties Of Jointed Rock Masses:Analysis Of Stress Waves

Waves play a crucial role in earthquakes,blasting and rock burst,for disaster mitigations and underground design-analysis,constructions.Therein we encounter natural rock masses with different joints that considerably affect constitutive properties.Extensive studies on static constitutive models for jointed rock masses are widely available.However,there are few studies of dynamic mechanical properties of jointed rock masses and especially their effects on stress wave propagation,even though for homogeneous rocks,which are devoid of interfaces,sufficient experimental and theoretical studies are at hand.This research focuses on theoretical formulations and experiment determinations of wave effects on jointed rock masses.On rock masses with and without joints,to determine dynamic compressive strengths SHPB(Split Hopkinson Pressure Bar)impact tests were conducted with longitudinal impacts.Whereas,to detect energy dissipation characteristics associated with tensile strengths the SHPB Brazil tests were studied with diametrical loading.Finally,influences of joint morphology and joint angle on stress wave propagations were explored.The main achievements are shown as following.(1)The SHPB impact compression test was conducted on intact rock,and the friction factors of the end face were controlled by changing the smearing conditions of bars and specimen with lubricant.The effect of different end face friction conditions of the SHPB test results was analyzed.Numerical simulation was used to analyze and obtain the effect of end face friction on stress-strain relationship and dynamic compressive strength.The results of the study indicate that the greater the end face friction,the smaller the fragmentation of the granite sample and the greater the dynamic compressive strength exhibited by the rock sample.(2)Based on quasi-static uniaxial compression test and SHPB impact compression test,the rock stress-strain curve and its relationship with strain rate were analyzed.The ZWT dynamic constitutive model was improved,the Maxwell model which reflects the quasi-static features of rock were excluded,the nonlinear spring elements was replaced by a statistical damage model which meets the Weibull distribution,and the dynamic damage constitutive model of rock was established.The calculated results of the model were in good agreement with the experiment and could reflect the strain rate effect and stress-strain relationship of intact rock under different strain rates.(3)The geometric characteristics and mechanical properties of joints have important influence on the dynamic mechanical properties of jointed granite.To establish the dynamic constitutive model of jointed granites,the macro damages were used to describe the joints,and the macro and meso damages of the jointed granite under impact compressive loads were coupled and analyzed based on the Lemaitre strain equivalent hypothesis.A dynamic mechanics constitutive model of the persistent jointed granite which considers the macro damages(joint planes)and micro damages(micro-crack micro-holes)of the jointed rock mass was constructed.The calculated stress-strain curve of the model was in good agreement with the experiment results which reflects the influence of geometric characteristics and mechanical properties of the joint on the stress-strain relationship of the jointed rock mass under impact load.(4)Rough joint surface was made by using splitting test and scanned by using a laser scanner to obtain the threedimensional topographical features of the rough surface.Based on the fractal theory,the joint surface fractal dimension was used to characterize the roughness of the joint surface.SHPB impact test was conducted by using the rough joint granite specimens.The results showed that the larger the fractal dimension of joint surface,the smaller the transmission coefficient and the equivalent stiffness of the joint specimen.Cemented mortars with different slotted grooves was used to simulate jointed rock mass with different contact area and same joint roughness.The SHPB impact test was performed on the simulated jointed rock mas.The results showed that the smaller the contact area,the smaller the transmission coefficient and the equivalent stiffness of the joint specimen.(5)Tensile stresses predominantly cause the failure of rock mass in blasting and similar projects.Hence the importance of SHPB Brazil tests,there the strain gauges were attached along the loading direction and on the joint surface.The strain-time curve indicated whether the joint specimen satisfies the center cracking condition,and then the tensile strength was obtained accordingly for the joint specimens which satisfies the center cracking condition.Hence the energy method was used to study energy dissipation in joint granite specimens under the impact splitting test.The effects of impact velocity,incident wave loading time,joint angle,and joint morphology on the failure mode and energy dissipation during the impact process of jointed rock mass were analyzed.